When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue tend to align with this polarizing field. If they are not initially aligned precisely with the polarizing field, they will precess about the field at their characteristic Larmor frequency as a top precesses about the Earth's gravitational field if the top's spin axis is not initially aligned with the field. Leveraging these fundamentals of physics, the nuclear spins of the nuclei of the highly-prevalent hydrogen atom are often targeted to investigate substances, such as through imaging, spectroscopy, or other analysis techniques. However, active nuclei of other elements are occasionally used in various applications.
Regardless of the particular nuclei being targeted, at equilibrium, the individual magnetic moments of all the nuclei combine to produce a net magnetic moment M in the direction of the polarizing field (maximizing Mz component). If the substance is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) that is in the x-y plane and that oscillates near the Larmor frequency, the net aligned moment, M, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy, which precesses (rotates about the B0 field direction) in the x-y plane at the Larmor frequency. The typically-brief application of the B1 field that accomplishes the tipping of the nuclear spins is generally known as an “RF pulse.” The practical value of this phenomenon resides in the signal that is emitted by the excited spins after the excitation field B1 is terminated. There is a wide variety of measurement pulse sequences (“sequences”) in which this NMR phenomenon is exploited for imaging, spectroscopy, and other application.
Under highly inhomogeneous static magnetic fields, MRI has fundamental difficulties in both signal excitation and data acquisition. For example, the inhomogeneous magnetic field spreads the frequency distribution of spin signals to the wide frequency range, which makes it difficult to excite all spins in the field of view (FOV) using conventional radiofrequency (RF) pulses. Moreover, the inhomogeneous field is permanently on and thus cannot be reversed as with pulsed gradient fields. Therefore, MRI under inhomogeneous fields usually relies on spin echo signals because the spin echo sequence refocuses the spin dephasing caused by field inhomogeneity. However, use of a refocusing RF pulse with a higher flip angle for spin echo formation is not feasible in MRI on human subjects with inhomogeneous fields, since it also needs to cover the wide frequency range of spin isochromats; that may require impractically high RF peak power and specific absorption rate (SAR).
Additionally, increased access to MRI scanning can occur if the cost of the MRI system can substantially decrease. One approach to achieve this is to decrease the size of MRI magnets, but this comes at a cost of drastically reduced static field (B0) homogeneity, which can easily lead to >100 kHz off-resonance variation.
Thus, a need continues for systems and methods for performing clinical MRI in the face of inhomogeneous static magnetic fields.